Noise rejecting limiter system



June 11, 1940. v. D. LANDON 2,204,090

NOISE REJECTING LIMITER SYSTEM Filed July 30; 1938 3 sheets-Sheet 1 lnventor Vernon D. Landon dnorncg June 11, 1940. v. D. LANDON 2,204,090

NOISE REJECTING LIMITER SYSTEM Filed July 50, 1938 3 Sheets-Sheet 2 R. F. OUTPUT R. P. INPU'I' lnnentor Vernon D. L andon Qnorneg June 11, 1940. v LANDON 2,204,090

NOISE REJECTING LIMITER SYSTEM Filed July so, 1938 0 s Sheets-Sheet s can v3 IF "I? "/6 OMITTED CURVE /F' 167'. .li/M/TER l6 OMITTED.

INPUT l/V 17-M-6 VOLTS GRID VOL 777G137 P517727 CURRENT TIME VOL 7'19 GE MOD/F/L'D CUR VE WHEN Zhwentor Ve rnon D. Landon dttorneg Patented June 11, 1940 UNITED STATES PATENT OFFICE Vernon D. Landon, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 30, 1938, Serial No.222,141

8 Claims.

This invention relates to noise rejecting limiter systems such as are utilized to eliminate undesired electrical impulses from the output of radio receiver apparatus or the like, and has for its 6 principal object the provision of an improved system and method of operation whereby the signal output of the system is reduced to a low value or to zero in response to received impulses exceeding a predetermined level.

A distinguishing characteristic of the invention is the use of an electron discharge device which is provided with a cathode lead resistor and is operated with relatively high screen grid and relatively low anode voltages. Under these conditions, the signal input is amplified and the signal output increases up to a predetermined signal input level, after which it decreases due to the cathode resistor drop which renders the cathode potential more positive than that of the anode. It will become apparent that, although the control grid of the electron discharge device swings positive with respect to ground, the control grid does not draw current because at the same time the cathode is subjected to a positive potential.

In the operation of this system at high radio frequencies, it is found that the cathode voltage may lag due to the cathode capacity, and less efficient rejection of strong noise impulses may be thus produced. This difliculty is avoided by tuning out the cathode capacity by means of a cathode to ground inductor or the like, a capacitor being connected in series with the inductor to prevent short-circuiting the. direct current component through the cathode resistor.

While the system so far described functions during the signal rejecting period with an output current which is very low and represents a relatively small percentage of the signal carrier fundamental, this output current may be readily caused to assume a substantially zero value during the signal rejecting periods. One suitable method of accomplishing this result is to interconnect the cathode and anode of the electron discharge device through a resistor or the equivalent, and to apply the signal to'the electron discharge device input circuit through an amplifier which also functions as an anode current cut-off limiter. As will be understood,. the electron discharge device or rcjector limiter is especially useful in connection with high radio frequency currents for the reason that only relatively low impedances are involved.

The invention will be better understood from the following description, considered in connection with the accompanying drawings, and its scope is indicated by the appended claims.

Referring to the drawings:

Figure l is a wiring diagram of a radio receiver to which the invention has been applied.

Figure 2 is a wiring diagram of the electron discharge device or rejector limiter which is'included in the system of Fig. 1,

Figure 3 illustrates the static grid voltage-plate current characteristic of the device of Fig. 2,

Figure 4 illustrates the radio frequency inputoutput characteristic of the device of Fig. 2,

Figure 5 illustrates the rejector limiter as fed through a plate cut-off limiter and as provided with a cathode-anode resistor for minimizing the carrier fundamental component in the output circuit during the rejection periods,

Figures 6 and '7 are explanatory curves relating to the operation ofthe system of Fig. 5, and

Figure 8 illustrates the cathode to ground re- 1 sister as shunted by a reactor for neutralizing the cathode capacity of the rejector limiter.

The radio receiver of Fig. 1 includes an antenna ii a plurality of wave traps I I, I2 and IS, a broadly tuned radio frequency channel section including the amplifier stages l4 and I5, a rejector limiter IS, a sharply tuned radio frequency channel section including amplifier stages l1 and I8, a detectorlS, an audio frequency channel section including amplifier stages 20 and 2|, and a loud speaker 22.

As stated in my copending application Serial No. 210,581, filed May 28, 1938, and assigned to the same assignee as the present application,

it is desirable that the amplifier stages preceding a limiter be broadly tuned and that the amplifier stages following the limiter be sharply tuned. In order to avoid the application to the limiter of broadcast signals from stations adjacent to the desired station, it is proposed by the aforesaid application to interpose in the antenna output leads a plurality of wave traps which are so ganged or mechanically coupled with the tuning elements of the sharply tuned radio frequency channel section that each unwanted broadcast signal is prevented from reaching the input of the broadly tuned channel section, thus allowing the broadly tuned channel section ahead of the limiter to cover the full width of the broadcast band except for the signal ranges excluded by the wave traps.

Insofar as the wave traps ll, l2 and IS, the broadly tuned channel section I4 and I5, and the sharply tuned channel section I! and I8 are concerned, the radio receiver of Fig. l is similar to that of the aforesaidapplication. Thus, in both cases, the wave trap switching means and the sharply tuned channel section and detector tuning elements are so controlled by a single member (23 in the present application) as to minimize the unwanted broadcast signals which would otherwise pass through the broadly tuned channel section and tend to saturate the limiter.

Power for operating the receiver of Fig. 1 is derived from a suitable direct current source, shown as a bleeder resistor 24 provided with terminals- 25 to 29 from which different voltages are applied to the various electrodes of the different stages.

It should be particularly noted that the anode 3B of the rejector limiter It has a relatively low voltage applied to it through the terminal 28, that a relatively high voltage is applied to the screen grid 3! of the limiter iii through the terminal 21, and that an unbypassed resistor 32 is connected between the limiter cathode 3S and the ground terminal 29. Signal impulses are applied to the control grid 43 of the limiter through a coupling transformer and the limiter output is supplied to the sharply tuned channel section through a coupling transformer 35, the suppressor grid 36 of the limiter l6 being connected to the low voltage primary terminal of the transformer 35 to afford a screening action which somewhat increases the output current.

As indicated more particularly in Fig. 2, the resistor 32 may have a resistance of the order of 6000 ohms, the potential applied to thescreen grid 3| may be of the order of 37 volts, and the potential applied to the anode 39 may be of the order of 6 volts. While these values have been found to give satisfactory results, it should be understood that they are not critical, but may be varied to suit the conditions under which the rejector limiter I6 is operated.

The static grid potential-output current char acteristic of the rejector limiter i5 is indicated by Fig. 3. It is desirable that the operating bias be such as to produce operation at the center of the positive slope of the curve as indicated by the line 31. In the operation of the system, a bias potential measured from grid to ground of the order of zero volts is found to be satisfactory, although this value is not critical, but may be varied to suit difi'erent operating conditions.

As the grid potential is made more positive the plate current increases in the normal man ner until the cathode potential approaches and passes the plate potential, at which point the plate current decreases becoming and remaining substantially zero for large positive grid excursions. The screen current continues to rise for positive grid voltages. The current which would ordinarily go to the plate goes to the screen instead, when the cathode becomes positive with respect to the plate.

The grid is never positive with respect to the cathode in the region of the curve illustrated in Fig. 3. The abscissa is the voltage from grid to ground.

As indicated by the RF input-RF output char acteristic of Fig. 4, the rejector limiter it output increases up to the point 38, and thereafter rapidly decreases to a relatively low value, due to the cathode being driven more positive by the increased potential drop of the resistor 32. While the rejector limiter output at the fundamental frequency of the carrier does not attain a zero value, this small percentage of fundamental frequency may not be objectionable in some cases.

If desired, it may be eliminated by modifying the rejector limiter connections, as shown by Fig. 5.

In this modification, a resistor M, connected between the anode 30 and the cathode 33 of the rejector limiter I6, serves to feed into the limiter output circuit a potential whereby the fundamental component is reduced to zero or completely cancelled in the limiter output circuit.

Thus, as indicated by the input-output characteristic of Fig. 6, the fundamental component of the output is reduced to zero in the region of the point 45 and tends to rise with increased input, as indicated by the broken line section 46.

This rise in the fundamental component with increased output, however, may be prevented by interposing between the broadly tuned channel section output and the rejector limiter iii a plate current cut-01f limiter which is provided with a cathode resistor 48 and is coupled to the control grid 43 of the limiter 16 through resistors 49 and 59 and a capacitor 5|.

The various electrical relations involved under these conditions are more clearly indicated by the curves of Fig. 7 which, taken with the accompanying legends, are self-explanatory. It will be noted that the full line fPlate current curve indicates the presence of a fundamental component for the reason that the peaks are unevenly spaced, and that this condition is corrected by the broken line modification of the full line curve, the broken line parts indicating how the full line plate current curve is modified by the insertion of the resistor M. Otherwise stated, the resistor 64 so modifies the plate current wave shape that the fundamental of the carrier voltage applied to the grid 43 is completely cancelled during the rejection period.

When operated at high or radio frequencies, the cathode voltage of the rejector limiter [5 tends to lag because of cathode capacity. This is likely to result in less efficient rejection of strong noise impulses. This difiiculty is avoided by connecting across the resistor 32 (Fig. 8) an inductor 4'5, a by-pass capacitor 48 being serially connected with the inductor to prevent shortcircuiting the resistor 32 to the direct current component.

While the combination of the electron discharge device lti with the cathode resistor 32,

and with the use of a relatively high voltage on the screen grid and a relatively low voltage on the anode, has been described in various environments, it will be apparent that this combination has other uses and that its novelty is not restricted to these particular environments.

I claim as my invention:

1. The combination of an electron discharge device including a cathode, an anode and a plurality of grids, means for applying electrical im- 1 pulses to one of said grids, means for applying a relatively low potential to said anode, means for applying a relatively high potential to another of said grids, and a resistor connected in series with said cathode whereby the cathode-anode if whereby the cathode-anode current'of said device is reduced when the level of said impulses increases above a predetermined value.

3. The combination, of an electron discharge device including a cathode, an anode and a plurality of grids, means for applying electrical impulses to one of said grids, means for applying a relatively low potential to said anode, means for applying a relatively high potential to another of said grids, a resistor connected in series with said cathode, and means for minimizing the funda mental frequency component of said impulses in the current delivered from said cathode and an ode, when said impulses exceed a predetermined value.

4. The combination of an electron discharge device including a cathode, an anode and a plurality of grids, means for applying electrical impulses to one of said grids, means for applying a relatively low potential to said anode, means for applying a relatively high potential to another of said grids, and a resistor connected between said cathode and anode for minimizing the fundamental frequency component of said impulse in the current delivered from said cathode and anode,

.when said impulses exceed a predetermined value.

device including a cathode, an anode and a plurality of grids, means for applying electrical impulses to one of said grids, means for applying a relatively low potential to said anode, means for applying a relatively high potential to another of i said grids, a resistor connected in series with said cathode, and means including an inductor con-- nected in shunt to said resistor for neutralizing the capacity effect of said cathode.

7. The combination of an electron discharge device provided with an anode, a plurality of grids, a cathode and an impedance device connected in the lead of said cathode, means including a plate cut-oil limiter for applying electrical impulses to one of said grids, means for applying a relatively low potential to said anode, and

means for applying a relatively high potential to another of said grids whereby the cathodeanode current of said device is minimized when the level of said impulses attains a predetermined value.

8. The combination of an electron discharge device provided with an anode, a plurality of grids, a cathode and an impedance device connected in the lead of said cathode, means including a plate cut-off limiter for applying electrical impulses to one of said grids, means for applying a relatively low potential to said anode, means for applying a relatively high potential to another of said grids, and means for balancing out of the current delivered from said cathode and anode the fundamental frequency component of said impulses, whereby said current is interrupted in response to a predetermined level of said impulses.

VERNON D. LANDON. 

